1. Field of the Invention.
The present invention relates to ultrasonic transducer systems for detecting distances to an ultrasonic reflecting surface, such as a system used for detecting levels of materials in a tank.
2. Description of the Prior Art.
Various ultrasonic distance measuring and level measuring systems have been advanced in the prior art, and in general the control of electroacoustic transducers which transmit the ultrasonic signals is well-known. Also, various logic device, microprocessor and computer controls for determining the time elapsed between the transmission of an ultrasonic signal and the receiving of a reflected signal are known.
For example, U.S. Pat. No. 4,114,441 shows a method and apparatus for measuring the height of filling materials on a surface, such as within a container, and provides means for determining the time interval between the end of the transmitted pulse from the acoustic transducer, and the end of the reflected or echo pulse.
A problem that has existed in the prior art is determining the levels when the tank is near a full condition. Attempting to detect valid reflected signals when the tank is nearly full is difficult because of a phenomena known as transducer "ring down". Essentially the "ring down" is the tendency of a transducer to continue to vibrate and transmit ultrasonic signals after the control signal that is used for initiating the transmitted signal has ceased. This "ring down" signal decreases in magnitude rather rapidly, but during the time that it is decreasing to a level below the threshold detection level, detection equipment will respond to such "ring down" signals if the signals are above a reference level and thus can indicate that a "ring down" signal is a reflected or return signal when in fact it is not. A common technique to avoid this problem is to blank out all return signals from the input of the detection system for a preselected period of time after initiation of a transmission. The preselected time is determined on various factors including the type of transducer that is used, but during this preselected time no valid returns can be sensed. If the tank is very near full, the return time for a reflected pulse is very short, and if the blanking technique is used, this results in a "dead band". Hence, determination of tank level in the near or very near full condition is not possible.